Terrain awareness systems, such as Terrain Awareness Warning Systems (TAWS), typically provide flight crews with information (e.g., alerts, warnings, displays of terrain data, etc.) related to detection of potentially hazardous terrain situations that pose or may pose a threat to the continued safe flight and landing of an aircraft and provide warnings or alerts in sufficient time for the flight crew to take effective action to prevent a controlled flight into terrain (CFIT). Terrain awareness systems generally have three alerting functions including a Ground Proximity Warning System (GPWS) alerting function, a Forward Looking Terrain Avoidance (FLTA) alerting function, and a Premature Descent Alerting (PDA) function.
Although the GPWS and TAWS have greatly reduced the controlled flight into terrain risk for aviation worldwide, both GPWS and TAWS have limitations. Neither of these systems detects the terrain or other obstructions ahead of the aircraft.
Instead, GPWS may differentiate the aircraft's altitude signals to detect abnormally high closure rates with terrain. However, highly discontinuous terrain profiles, such as a cliff, may not generate an alert in sufficient time to prevent an accident. In addition, non-fixed terrain features and non-fixed terrain threats such as, for example, aircraft or vehicular traffic on the runway, are also not readily determinable by typical ground proximity warning systems.
Similarly, the “look ahead” function of TAWS compares aircraft position data, based on a stored terrain map to calculate the aircraft's probable position relative to the terrain and determine whether a terrain collision threat exists. However, this system cannot detect collision threats due to obstructions not contained within the database (e.g. a construction crane). In addition, the integrity of the TAWS depends directly upon the integrity of the aircraft position data. Errors in aircraft position could reduce the warning time given the flight crew.
An alternative means of compliance with the FAA TAWS forward looking terrain alerting function requirements (which are currently defined in FAA Technical Standard Order ISO C115b) is a Radar-based TAWS (R-TAWS). Such a system uses airborne radar sensors (e.g., similar to the radar that is used for weather and wind shear detection) to detect terrain and/or obstacles that are potentially hazardous to the aircraft rather than or in addition to using a database.
However, the efficacy of an R-TAWS system may be impacted by the presence of intervening weather between an aircraft and a terrain obstacle. As such, it may be desirable to provide an R-TAWS attenuation alert.